Method and system for removing alkyl halides from gases

ABSTRACT

An alkyl halide such as methyl bromide can be effectively removed from a gas stream by passing the gas stream through a bed of adsorbent at a relatively low temperature to adsorb the alkyl halide onto the adsorbent, desorbing the alkyl halide at a higher temperature to produce a smaller volume of gas containing the alkyl halide in more concentrated form, and then reacting the alkyl halide with a nucleophile contained in a liquid phase by contacting this smaller gas volume with such liquid phase.

FIELD OF THE INVENTION

This invention relates to the removal and destruction of alkyl halidesfrom a gas. In particular, the invention provides a method as well as asystem for effectively and efficiently dealing with the large volumes ofmethyl bromide-contaminated air generated in connection with fumigationoperations.

BACKGROUND OF THE INVENTION

Many alkyl halides possess a degree of toxicity, sometimes very hightoxicity. For example, the toxicity of methyl bromide is so great thatit has been used for many years in the extermination of insects inmills, warehouses, vaults, ships, freight cars, imports and exports, andalso as a soil fumigant for use by growers of strawberries, tomatoes,and other crops. Other applications include treatment of ships to removeundesirable rodents and insects and the treatment of foods such asfruits (including dried fruits), grain, flour, nuts, and tobaccoproducts to remove potential pests. Additionally, methyl bromide hasbeen successful in fumigation against various microorganisms includingfungi and bacteria. Recently, methyl bromide has been advocated as themost effective agent against anthrax (Bacillus anthracis). Its virtuesinclude that it is nonflammable and not explosive, has a very highdiffusivity and permeability, and has been used safely for over 60years.

Unfortunately, release of methyl bromide into the atmosphere isgenerally accepted to cause ozone layer depletion that can result inincreased incidences of skin cancer. Direct release of methyl bromideinto the environment is also a concern to workers and bystanders who maybe harmed by its toxic effects. Thus, there is a need for methods ofdisposing of methyl bromide without releasing it to the atmosphere.There is also a general need for methods of rapidly and economicallyremoving volatile alkyl halides such as methyl bromide from gas streams.

In particular, methyl bromide is widely used for large scale quarantinefumigation of products such as produce and other agricultural goods,with the chambers utilized in such applications typically ranging fromabout 850 to about 7000 cubic meters (about 30,000 to about 250,000cubic feet) in size. The initial methyl bromide concentration employedin these chambers generally is from about 24 to about 128 grams percubic meter (about 1.5 to about 8.0 lbs/1000 cubic feet). CurrentUSDA-APHIS regulations for imports and exports require that the aerationstep needed to remove methyl bromide from a chamber following fumigationbe a minimum of four turnovers per hour of fresh air in order to reducethe concentration of methyl bromide below 5 ppmv (parts per million byvolume). Sweeping the methyl bromide out of a fumigation chamber toaerate the chamber in this manner using fresh air results in thegeneration of very large volumes of methyl bromide-contaminated air.Removing the methyl bromide from such large gas volumes in acost-effective manner has proven to be very challenging.

SUMMARY OF THE INVENTION

In one aspect of the invention, a method of removing an alkyl halidefrom a first gas volume is provided. This method comprises:

contacting the first gas volume with an adsorbent capable of adsorbingthe alkyl halide from the first gas volume within a first temperaturerange, thereby producing an alkyl halide-containing adsorbent;

contacting a volume of air that is smaller (preferably, much smaller)than the first gas volume (for example, the air volume may be less than20% or less than 3% or less than 0.5% of the volume of the first gasvolume) with the alkyl halide-containing adsorbent within a secondtemperature range that is greater than the first temperature range andeffective to desorb at least a portion of the alkyl halide from theadsorbent, thereby producing a second gas volume containing the alkylhalide and having a volume less than the volume of the first gas volumeand an alkyl halide concentration greater than the alkyl halideconcentration of the first gas volume; and

contacting the second gas volume with a liquid phase comprising waterand a nucleophile capable of reacting with the alkyl halide to produce apurified gas stream.

In another aspect, the present invention provides a system for removingan alkyl halide from a gas volume, the system comprising:

a) an adsorption/desorption unit comprising an adsorbent capable ofadsorbing the alkyl halide from the gas volume; and

b) a reactor assembly comprising a reaction vessel containing a liquidphase comprised of water and at least one nucleophile.

The present invention thus affords a practical method for removing anddestroying fumigation agents such as methyl bromide from fumigationaeration streams or discharge gases (even where such streams ordischarge gases are quite large in volume) which is relativelyinexpensive, effective, safe and executable on-site such thattransportation of methyl bromide-containing wastes to an off-sitefacility is not necessary. By use of the inventive process, rapiddegradation (e.g., within 2 to 24 hours) of an alkyl halide-contaminatedgas volume can be achieved, thereby permitting fumigation chamber cycletimes to be kept advantageously short and increasing product throughputthrough the chambers.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of a system employing a methodaccording to one exemplary embodiment of the invention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

One step of the present invention comprises contacting a first gasvolume containing one or more alkyl halides with an adsorbent capable ofadsorbing the alkyl halide(s) within a first temperature range, therebydepositing or condensing the alkyl halide(s) onto and within theconfines of the adsorbent and producing an alkyl halide-containingadsorbent. The alkyl halide-containing first gas volume may be obtainedfrom any suitable source. However, the method of the present inventionis especially useful for purifying methyl bromide-contaminated gasvolumes generated from fumigation operations (i.e., fumigation aerationstreams or discharge gases). For example, the first gas volume may beobtained by providing an enclosure defining a confined space to befumigated (e.g., a fumigation chamber, typically having a volume of from27 to 8500 cubic meters or about 1000 to about 300,000 cubic feet ormore), placing a product (e.g., produce such as fruits or vegetables orwood products that require fumigation) within the enclosure, sealing theenclosure, introducing an amount of an alkyl halide (e.g., methylbromide) effective to fumigate the product into the enclosure to producea gaseous atmosphere comprised of air and alkyl halide (in the case ofmethyl bromide, typically at a concentration of 24 to 128 grams percubic meter or 1.5 to 8.0 lbs/1000 cubic feet of fumigation volume),allowing the gaseous atmosphere to remain in contact with the productfor a length of time effective to fumigate the product to a desiredextent, and flushing the gaseous atmosphere from the enclosure usingadditional air. The concentration of alkyl halide in the gas streaminitially exiting from the enclosure will be relatively high, with thealkyl halide concentration in the gas stream exiting from the enclosurethereafter decreasing as fresh air is introduced into the enclosure andused to flush the confined space. In most instances, the decrease inalkyl halide concentration follows a classic exponential decay curve.Deviations from exponential decay concentration behavior can occur whenthe produce or other goods being fumigated have a very high affinity foralkyl halide.

The adsorbent may be any material or substance capable of effectivelyadsorbing the alkyl halide from the first gas volume when such first gasvolume is contacted with the adsorbent, preferably at a relatively lowtemperature (e.g., less than 35° C. or at normal ambient temperatures).Solid, particulate adsorbents are preferred. In one desirable embodimentof the invention, carbon (e.g., activated carbon particles) is employedas the adsorbent, although other suitable adsorbents may includezeolites (molecular sieves) and the like. The adsorbent may be containedin a vessel such as a column. For instance, the adsorbent may be in theform of a bed within a vessel arranged such that the first gas volumemay be introduced into the vessel and passed through the bed as a streamin either an upflow or downflow manner, thereby allowing the alkylhalide (which is typically in the vapor phase within the first gasvolume) to be adsorbed by the adsorbent. The gas stream exiting from thevessel after passing over or through the adsorbent has a reducedconcentration of alkyl halide as compared to the alkyl halideconcentration present in the gas stream being introduced into thevessel. Typically, the alkyl halide concentration is greatly reduced; incertain embodiments, the alkyl halide concentration in the exiting gasstream is below detectable limits. In other words, during operation ofthe adsorption step of the present invention, the alkyl halideconcentration at the outlet of the vessel is lower than the inletconcentration of alkyl halide.

The contacting of the first gas volume containing at least one alkylhalide with the adsorbent should be carried out within a temperaturerange that is relatively low. Preferably, the contacting temperature isset as low as possible within the economic constraints of the systembeing fumigated. In some instances, goods are fumigated at about 40° F.(ca. 4° C.) and thus the discharge gas stream removed from thefumigation chamber is already at a relatively low temperature. This lowtemperature is advantageous since the lower the temperature, the greaterthe affinity for the methyl bromide to become adsorbed on the adsorbent(e.g., carbon). While still lower temperatures would thus be even moreadvantageous with respect to the adsorption efficiency, the energy costsassociated with cooling large volumes of fumigation aeration gases wouldrender the process uneconomic.

The amount of adsorbent utilized should be selected to be sufficient toadsorb substantially all (e.g., at least 90% or at least 95% or even atleast 99%) of the alkyl halide present in the first gas volume. Thefirst gas volume may be recycled or otherwise brought into repeatedcontact with the adsorbent in order to reduce the alkyl halideconcentration to the desired level. For example, the gas stream exitingthe vessel containing a bed of the adsorbent may be recirculated backthrough the vessel one or more times, provided the adsorbent bed hassufficient capacity to ensure that the alkyl halide concentration doesnot rise to an unacceptable level. Typically, the alkylhalide-containing adsorbent contains from about 0.5 to about 5 weightpercent alkyl halide.

Desorption of the alkyl halide adsorbed on the adsorbent is accomplishedby contacting a volume of air with the alkyl halide-containingadsorbent. This volume of air is smaller (typically, much smaller) thanthe volume of the first gas volume (e.g., the desorption air volume maybe less than 20% or less than 3% or less than 0.5%, but generally willbe at least 0.1%, of the volume of the first gas volume). During thedesorption air contacting, the temperature is maintained within a secondtemperature range that is greater than the first temperature range andeffective to desorb at least a portion of the alkyl halide (preferablyat least 70% and more preferably at least 85% or at least 99% of thealkyl halide) from the adsorbent, thereby producing a second gas volumecontaining the alkyl halide and having a volume less than the volume ofthe first gas volume and an alkyl halide concentration greater than thealkyl halide concentration of the first gas volume. Advantageously, thepresent process may be operated such that the second gas volume which isproduced is less than 20% or less than 3% or even less than 0.5% of thevolume of the first gas volume. Typically, however, the volume of thesecond gas volume is at least 0.1% of the volume of the first gasvolume. It is convenient and advantageous to carry out such contactingby introducing a stream of air into a vessel containing the alkylhalide-containing adsorbent, allowing the air stream to pass over orthrough the adsorbent (for example, in an upflow or downflow manner),and then withdrawing the air stream containing the desorbed alkyl halidefrom the vessel to yield the second gas volume. During this contactingand desorption step, the outlet concentration of alkyl halide willgenerally be greater than the inlet alkyl halide concentration observedduring the adsorption step. The withdrawn stream may be recycled backinto contact with the alkyl halide-containing adsorbent so as to furtherdecrease the amount of alkyl halide still retained on the adsorbent. Aswith the adsorption step, the optimum temperature for desorption willdepend upon a number of variables, but typically the desorptiontemperature is at least 50° C., but no more than 300° C. (preferably, nomore than 150° C.), higher than the adsorption temperature. In certainembodiments of the invention, the desorption temperature is less than150° C., e.g., within the temperature range of 80 to 120° C. or withinthe temperature range of 90 to 110° C.

The step of contacting the second gas volume (containing desorbed alkylhalide) with a liquid phase comprising water and a nucleophile capableof reacting with the alkyl halide to produce a purified gas stream canbe carried out using any suitable procedure. The contacting may beperformed on either a batch or continuous basis. Preferably, thecontacting is conducted in accordance with the methods described inpublished United States application No. 2006-0088462, incorporatedherein by reference in its entirety.

For example, bubbles of gas may be passed through an aqueous solution ofa nucleophile, optionally containing an organic compound, which has beenfound to increase the effectiveness of alkyl halide removal. The bubblesrise through the aqueous solution, during which time the alkyl halide isconverted to relatively nonvolatile materials, which may then becollected for use or merely disposed of.

The invention will next be illustrated with reference to the figure. Thefigure is intended to be illustrative rather than limiting and isincluded herewith to facilitate the explanation of the presentinvention. The figure is not to scale, and is not intended to serve asan engineering drawing.

FIG. 1 shows in schematic form a system suitable for practicing themethod of the present invention, according to one exemplary embodiment.FIG. 1 does not show all valves, pumps, blowers, flow meters, analyzersand the like which might be employed in such a system, but the skilledartisan will recognize from the following description that the placementand design of such features are flexible and that the system can readilybe configured in many possible ways to achieve effective andsatisfactory results. The system comprises an enclosure 10 defining aconfined space 11 to be fumigated and capable of being sealed, such as afumigation chamber. A product 12 requiring fumigation, such as aquantity of produce, is present within enclosure 10. Enclosure 10 issealed and an amount of methyl bromide effective to fumigate the productis introduced via line 13 into enclosure 10 to produce a gaseousenvironment within confined space 11 comprised of air and methylbromide. The gaseous environment is contacted with product 12 for alength of time effective to fumigate product 12 to the desired extent.Thereafter, fresh air is introduced into enclosure 10 via line 14 andthe gaseous environment is withdrawn from enclosure 10 via line 15 toproduce a first gas volume containing air and methyl bromide. The firstgas volume comprises the gaseous environment as well as the fresh airused to flush residual methyl bromide from the enclosure 10.

The first gas volume may be passed directly into adsorption/desorptionunit 16 or, alternatively, may be stored temporarily in a tank or othergas-containing structure prior to being introduced intoadsorption/desorption unit 16. Adsorption/desorption unit 16 comprises ahollow column 17 containing a bed 18 of activated carbon particles 19.The first gas volume is permitted to flow through bed 18 underconditions effective to cause adsorption of the methyl bromide onto theactivated carbon particles 19. For example, the temperature at which thefirst gas volume is contacted with the activated carbon particles isgenerally relatively low (e.g., less than 50° C., or within the range of−20 to 35° C., or at typical ambient or room temperatures). Temperaturecontrol within adsorption/desorption unit 16 may be effected by anysuitable technique, such as employing a jacketed column cooled and/orheated by circulating water or other liquid or by directly heating orcooling a gas stream before it is introduced into theadsorption/desorption unit 16. The gas stream exiting fromadsorption/desorption unit 16 via line 20 thus has a reducedconcentration of methyl bromide as compared to the stream of the firstgas volume entering the adsorption/desorption unit 16. If it is desiredto limit the total volume of discharge gas removed from enclosure 10 orthe amount of aeration air introduced into the system, the exiting gasstream can be introduced back into enclosure 10 to assist in sweepingout residual methyl bromide from confined space 11. In yet anotherembodiment of the invention, adsorption/desorption unit 16 is operatedsuch that the exiting gas stream has a low enough concentration ofmethyl bromide such that it can safely be vented directly into theatmosphere. Although a single adsorption/desorption unit 16 is shown inFIG. 1, another embodiment of the present invention would be to use twoor more such units which may be connected in parallel and/or in sequence(not shown).

Once adsorption of methyl bromide from the first gas volume onto theadsorbent has proceeded to the desired extent, desorption of the methylbromide may be initiated by closing line 20, opening line 21 (leading toreactor assembly 22), and introducing a stream of fresh air or other gashaving a suitably low methyl bromide concentration intoadsorption/desorption unit 16 via line 31. The temperature of the bed 18of activated carbon particles 19 is also increased to within a rangeeffective to promote relatively rapid methyl bromide desorption. Thefresh air stream is passed through bed 18 and thus brought into contactwith activated carbon particles 19, which contain adsorbed methylbromide. The desorption conditions (adsorption/desorption unit 16temperature, gas flow rate, volume of fresh air introduced, etc.) arecontrolled such that a second gas volume is generated which containsmethyl bromide in a concentration greater than that of the first gasvolume and which also has a volume less than the volume of the first gasvolume. In effect, by operation of the present invention, the relativelylarge gas volume generated by venting and purging the enclosure 10containing methyl bromide in relatively diluted form is converted to arelatively small gas volume containing methyl bromide in relativelyconcentrated form. Thus, by concentrating the methyl bromide in a muchsmaller gas volume, equipment costs associated with gas handling and thesize of the scrubber system that are directly proportional to the volumeof gas being handled (and that are nearly independent of the methylbromide concentration in the gas stream) are significantly reduced.

A stream containing the second gas volume is withdrawn fromadsorption/desorption unit 16 via line 21 and delivered to reactorassembly 22. A purified gas stream exits from reactor assembly 22through line 23. This purified gas stream may be recycled back toadsorption/desorption unit 16, or it may be released through a vent intothe atmosphere (if the alkyl halide concentration is suitably low), ordelivered into a product tank or other enclosure or into another reactorassembly (scrubber). By recycling the purified gas stream back throughbed 18 in adsorption/desorption unit 16, a higher level of alkyl halideremoval may be obtained. Although a single reactor assembly 22 is shownin FIG. 1, two or more may be used, and they may be connected inparallel and/or in series (not shown).

Reactor assembly 22 comprises a reaction vessel 24 containing a liquidphase 25. Reaction vessel 24 may be of any convenient shape andappropriate material of construction. In the embodiment shown in FIG. 1,the gas stream which comprises the second gas volume containing thedesorbed methyl bromide passes into liquid phase 25 through a gasdisperser 26, for example a glass frit that provides introduction ofsmall bubbles of feed gas into the liquid to enhance the overallgas-liquid mass transfer rate. Other types of gas dispersers may also beused, for example a pipe with holes in it, or a plate with holes in it,or any other device known in the art to convert the gas stream intosmall bubbles. Bubbles 28 rise through a liquid column 29 through liquidphase 25 until they reach the upper surface 30 of liquid phase 25,during which time contact is made such that the alkyl halide can rapidlybe carried into the liquid phase 25. It is preferred that the bubbles 28be small, to maximize the gas-liquid surface area and thereby increasethe rate at which alkyl halide is carried into liquid phase 25 to thepoint where gas-liquid mass transfer is not the rate-limiting step inthe reaction of alkyl halide with nucleophile.

Suitably small bubbles may be provided by any means known in the art,but they are conveniently provided by the use of porous tubes (spargers)having pores between 1 and 200 microns across at their widest point.Typically, the pores will be between 10 and 50 microns across. In oneembodiment, the majority of the pores are within the range specified.The holes are typically roughly circular in shape, although other shapescould be employed. The spargers are typically situated such that thereis a relatively unobstructed or free flow of bubbles through the aqueousphase containing the nucleophile. If the bubbles collide with each otherin a manner where they lose their individual integrity and thus coalesceand create larger bubbles, as is the case when the volume of gas passedthrough the sparger is too great or the spargers are too close to eachother, significantly reduced removal efficiencies may be encountered dueto the decreased mass transfer area of methyl bromide into the liquidphase. The design issue becomes providing enough gas-liquid contact areavia the creation of small, finely divided bubbles to transfersignificant amounts of reactive alkyl halide gas to the liquid phase.One of the factors determining the bubble size is the size of the poresin the sparger tubes. When the pores are too small the correspondingincreased pressure drop may be very large thus requiring gas compressionof large volumes of gas that unnecessarily increases the processingcost.

FIG. 1 does not show an agitator, although one may be used. However, thepresent invention does not require any mechanical stirring, but takesadvantage of the turbulence created in the liquid phase due to theintroduction of the gas through the small openings in the gasdisperser(s). Liquid phase 25 may be recycled or discarded when thenucleophile has been depleted due to reaction with the alkyl halide.Liquid phase 25 may be discharged to a wastewater treatment plant (WWTP)suitably equipped to handle high dissolved salt concentrations.Regeneration or replenishment of the liquid phase may also be carriedout, either periodically or continuously, by, for example, withdrawingportions of the liquid phase (after it has been contacted with thesecond gas volume) and introducing fresh amounts of nucleophile andpossibly other components of the liquid phase to replace materialsconsumed by reaction with the alkyl halide contained in the second gasvolume.

In some applications, for example where alkyl halide levels are to bereduced to an especially low level, it may be desirable to connect twoor more reactor assemblies (scrubbers) in series, such that purifiedgases exiting a scrubber are further purified by subsequent passagethrough another. On the other hand, in some applications it may bedesired to rapidly purify a large volume of gas, in which case two ormore scrubbers may be used in parallel. Combinations of series andparallel arrangements may also be practiced according to the invention,using multiple scrubbers.

Typically, the distance 29 from the upper end of gas disperser 26 toupper surface 30 of the liquid phase 25 is at least 15 cm (6 inches) andmore typically at least 31 cm (12 inches). The distance is typically atmost 310 cm (120 inches) and more typically at most 244 cm (8 feet).However, greater liquid depths can be used, as long as the blower (orother device used to introduce the alkyl halide-containing gas streaminto the liquid phase) has sufficient capacity to introduce gas at thedesired flow rate and pressure. Thus, no real upper limit for liquiddepth exists other than that resulting from blower capability, availablespace, and other practical limitations.

The inventors have found that the rate at which the gas stream entersthe reactor assembly (scrubber) affects the degree of completeness withwhich the alkyl halide is consumed, with too high a rate tending todecrease the degree of alkyl halide destruction. One suitable measure ofthe rate of gas flow relative to the size of the scrubber is thesuperficial gas velocity, which may be calculated by dividing thevolumetric flow rate of gas into the scrubber by the averagecross-sectional area of the scrubber. An acceptable superficial gasvelocity for a given situation depends inter alia upon the type andconcentration of alkyl halide in the gas stream entering the scrubber,the type and concentration of nucleophile employed, the amount and type(if any) of soluble organic compound in the liquid phase, the size ofthe bubbles produced by the gas disperser(s), the distance that thebubbles travel through the liquid phase, the temperature of the liquidphase, and the desired level of alkyl halide removal from the gasstream. For example, when scrubbing methyl bromide from an air streamwith thiosulfate in the presence of PEG 200, using a gas disperserhaving approximately 20-μm pores and a 31 cm (12-inch) travel of theresulting rising bubbles, a superficial gas velocity may typically be atmost 75 cm/min (2.5 ft/min), and more typically will be in the range of15 to 33 cm/min (0.5 to 1.1 ft/min).

The liquid phase contains water and at least one nucleophile, but mayalso include dissolved materials such as co-solvents, and of courseproducts formed by the nucleophilic reaction of the alkyl halide and thenucleophile. The liquid phase is typically essentially free of suspendedundissolved material, but this is not required. The term “nucleophile”as used herein means an anion or molecule having a high electron densitywhich is accessible for reaction with another molecule by displacementof a leaving group, typically an anion such as halide. Due to thepresence of a good leaving group (halide anion), alkyl halides can takepart in nucleophilic substitution reactions with nucleophiles, suchreactions typically (but not necessarily) being of the bimolecular(S_(N)2) type.

Many neutral and anionic nucleophiles can participate in nucleophilicsubstitution reactions with alkyl halide. A non-limiting list of anionssuitable for use as nucleophiles according to the invention includes thefollowing and their derivatives: cyanide (CN⁻), thiocyanate (SCN⁻),cyanate (OCN⁻), bisulfide (HS⁻), sulfide (S²⁻), carbonate (CO₃ ²⁻),bicarbonate (HCO₃ ⁻), thiocarbonates (monothio, dithio, and trithio),azide (N₃ ⁻), sulfite, bisulfite, alkyl, aryl, or aralkyl thiolate,nitrite, nitrate, phosphates (mono and di hydrogen phosphates plusphosphate), thiophosphates, biselenide (HSe⁻), selenide (Se² ⁻),(substituted and non-substituted) benzenesulfonate, chloride, bromide,fluoride, iodide, thiosulfate, chlorate, hypochlorite, malonate,carboxylates such as trichloroacetate (CCl₃COO⁻), dichloroacetate,chloroacetate, terephthalate, adipate, lactate, m-chloroperbenzoate,formate, acetate, acrylate, propionate, butyrate, benzoate, furoate,oxalate, phthalate, hydrogen phthalate, silicates, bromate, periodate,performate, and phenolate, cresolate, and catecholate. Suitable neutralnucleophiles may include for example ammonia and primary, secondary, andtertiary amines, where the substituents on nitrogen may be anycombination of alkyl, aryl, and aralkyl groups, and phosphines analogousto such amines. In this context, the term “derivative” means a compoundthat contains one of the nucleophilic groups listed above.

Particularly suitable nucleophiles for use according to the inventioninclude compounds containing sulfur or nitrogen at the nucleophiliccenter. As used herein, the term “nucleophilic center” means that atomwhich becomes bonded to the alkyl halide residue by virtue of thenucleophilic reaction. Specific examples of suitable sulfur nucleophilesinclude aliphatic and, preferably, aromatic thiols and their salts,aliphatic and aromatic disulfides and polysulfides, sulfide anion,bisulfide anion, thiosulfate anion, sulfite or bisulfite anion, andthiocyanate anion. In one exemplary embodiment of the invention, thenucleophile comprises at least one of sodium sulfide and sodiumbisulfide at a concentration of from about 0.1 wt % to the saturationlimit in the liquid phase. When sulfur nucleophiles are used, it may beadvantageous to oxidize the resulting reaction products, for examplewith sodium hypochlorite, to convert them to materials having less odor.

Other suitable nucleophiles are alkoxides, carboxylates, hydroxide, andselenium analogs of sulfur nucleophiles.

When a precursor species must be ionized to become a highly reactivenucleophile, for example when a hydroxy compound or thiol or carboxylicacid must be converted to the corresponding anion, a pH-adjusting agentis used in such an amount as to ensure that the pH is raised to a levelsufficient to ionize the chemical species, namely by removing a protonfrom the species and generating a negatively charged species in theliquid phase. The required pH is dependent on the nature of thenucleophile, namely whether its conjugate acid is a strong or weak acid.For example, if the nucleophile is the anion of a weak acid, arelatively higher pH may be required in order to produce a sufficientconcentration of the anion. Conversely, when the chemical speciesalready exists as a nucleophilic anion or as a neutral compound that canact as a nucleophile, no pH-adjusting agent may be needed. When a pHadjusting agent is needed, the particular amount of the agent or basewill vary depending on process conditions, but can be optimized easilyby altering the concentration and determining its effect on yield,bearing in mind the ranges of excess molar concentrations set forthabove.

According to the present invention, a pH-adjusting agent (if needed toproduce suitable quantities of nucleophile) is used in an amountsufficient to provide an excess molar concentration of base in the rangebetween −0.99 and 1.0, preferably between −0.25 and 0.5, more preferablybetween stoichiometric and 0.25, and most preferably between 0.01 and0.1. As used herein, the term “stoichiometric” means the amount of baseindicated by a balanced chemical equation to be necessary to convert allof the precursor species to the desired nucleophile. Thus, the “excessmolar concentration of base” is the amount of base actually in thesystem above that which would be stoichiometrically required toneutralize ionizable hydrogen atoms, and is expressed herein as thedifference between the actual concentration of base and thestoichiometric concentration divided by the stoichiometricconcentration. Thus, a negative value of excess molar concentration ofbase contemplates that less than the stoichiometric amount of base isused.

A suitable pH for purposes of the invention is one at which anucleophilic anion is present and is at least partially soluble in theliquid phase, typically from pH 7 to 13.5. However, certain embodimentsof the present invention may provide sufficient amounts of nucleophileeven at lower pH values, even as low as a pH of about 1, depending onthe nucleophile used.

It should be recognized that the pH as used herein refers to the pH inthe liquid phase. The pH adjusting agent may be added to the liquidphase prior to contacting the gas stream containing alkyl halide, orafterwards. Any of a number of suitable pH adjusting agents may be used,but some typical ones are sodium hydroxide, potassium hydroxide,magnesium hydroxide, calcium hydroxide, lithium hydroxide, ammoniumhydroxide, magnesium carbonate, calcium carbonate, tetraalkyl ammoniumhydroxides, sodium and potassium carbonates, bicarbonates, phosphates,similar salts, and mixtures thereof.

The liquid phase may also contain a water-soluble organic compound, andthe presence of such compounds has been found in some cases to increasethe rate and/or completeness of alkyl halide destruction. For example,the addition of water soluble organic compounds such asN-methylpyrrolidone, dimethyl formamide, dimethyl sulfoxide (DMSO), andpoly(ethylene glycol) to the water phase has been shown to improve thelevel of removal of methyl bromide. Without wishing to be bound by anyparticular theory or explanation, it is believed that the water solubleorganic compound increases alkyl halide solubility by decreasing thepolarity of the liquid phase, and that this increases the rate ofreaction between the alkyl halide and the nucleophile. Water-solubleorganic compounds may constitute between 1 and 99 wt % of the liquidphase, more typically between 1 and 25 wt %. In some embodiments, theorganic compound is relatively nonvolatile, by which is meant it doesnot boil below 125° C. In some embodiments of the invention, the organiccompound is a polyglycol according to the formula H—(OCH₂CHR)_(n)—OH,wherein n is an integer from 1 to 20 and R is H or CH₃. One usefulexample is tetraethylene glycol.

Preferred nucleophilic reaction conditions for the destruction of alkylhalides depend on a number of factors, including the specificnucleophilic species used, and the organic substrate used. In general,the time and temperature should be selected to cause the reaction toproceed rapidly. As is well known, the choice of temperature is dictatedby the kinetics of the reaction and the solubility of alkyl halide inthe reaction medium. Reactions that occur more slowly are preferably runat higher temperatures. Lower reaction temperatures may however besuitable or even preferable in some situations, provided only that thereaction rate of alkyl halide be sufficiently fast to achieve thedesired degree of removal. Typical suitable temperatures are from −3° C.to 105° C., more typically from 2 to 40° C., and most typically from 5to 30° C.

The reactor assembly (scrubber) may be run at approximately atmosphericpressure, i.e., atmospheric pressure plus the incremental additionalpressure generated by the head of liquid over the gas disperser(s). Itmay also be operated at pressures well above atmospheric, and there isno known limit to how high a pressure may be used provided that thesafety of the processing equipment is not compromised by pressures thatexceed manufacturers' recommendations. Higher pressures may increase therate of reaction, and may be especially useful in cases where there is arelatively high concentration of alkyl halide and a correspondinglylower level of diluent gas (e.g., air) in the feed, since the cost ofcompressing and decompressing the feed may be less in such a situation.Higher pressures may also be beneficial when a higher scrubber reactiontemperature is desired, with the higher pressure making it possible toreduce loss of water or other volatile components.

In one embodiment of the invention, the adsorption and desorption stepsmay be repeated at least once so as to even further concentrate thealkyl halide prior to introducing the gas stream containing the alkylhalide into the reactor assembly (i.e., prior to contacting such gasstream with the liquid phase comprising water and one or morenucleophiles). For example, the following series of steps may bepracticed:

a first gas volume containing an alkyl halide is provided;

the first gas volume is contacted with a first portion of an adsorbentcapable of adsorbing the alkyl halide from the first gas volume within afirst temperature range, thereby producing a first alkylhalide-containing adsorbent;

a first volume of air that is smaller in volume than the first gasvolume is contacted with the first alkyl halide-containing adsorbentwithin a second temperature range that is greater than the firsttemperature range and effective to desorb at least a portion of thealkyl halide from the first portion of adsorbent, thereby producing asecond gas volume containing the alkyl halide and having a volume lessthan the volume of the first gas volume and an alkyl halideconcentration greater than the alkyl halide concentration of the firstgas volume;

the second gas volume is contacted with a second portion of an adsorbentcapable of adsorbing the alkyl halide within a third temperature range,thereby producing a second alkyl halide-containing adsorbent;

a second volume of air that is smaller in volume than the first volumeof air is contacted with the second alkyl halide-containing adsorbentwithin a fourth temperature range that is greater than the thirdtemperature range and effective to desorb at least a portion of thealkyl halide from the adsorbent, thereby producing a third gas volumecontaining the alkyl halide and having a volume less than the volume ofthe second gas volume and an alkyl halide concentration greater than thealkyl halide concentration of the second gas volume; and

the third gas volume is contacted with a liquid phase comprising waterand a nucleophile capable of reacting with the alkyl halide to produce apurified gas stream.

EXAMPLES

The adsorption and desorption steps of the method of the presentinvention are demonstated by the following example.

A stainless steel insulated column that is 2.5 cm in diameter was filledwith activated carbon particles to a depth of 127.4 cm (carboncharge=311.8 g; carbon density=0.457 kg/m³). Temperature control withinthe column was maintained with circulating and pressurized hot water inan external heat exchanger jacket that enveloped the full length of thecarbon bed. The inlet, outlet and jacket temperatures were measured with3-wire RTDs. Inlet and outlet temperature probes were welded into thepipe so that they protruded directly into the gas streams. A jackettemperature probe was placed on the surface of the jacket, underneath alayer of fiberglass insulation. Adsorption and desorption air flow rateswere measured using factory calibrated mass flow meters (Cole-Parmer).The outlet concentration of methyl bromide in air was measured usinginfrared absorption with a methyl bromide analyzer supplied by SpectrosInstruments (Hopedale, Mass.). Calibration gas standards of 1.50. 0.76and 0.18 volume percent methyl bromide in air (Scott-Marin, Riverside,Calif.) were used to check the calibration of the IR analyzer. Using themass flow meters, the calibration standards were fed to the column in amanner to simulate the exponential concentration decay flow loading ofmethyl bromide onto the carbon column (inlet gas concentration to thecarbon bed was based on a typical exponentially decaying feed streamexiting a fumigation chamber). All of these instruments were connectedto a National Instruments Labview® data acquisition system connected toa personal computer. Data points for all instruments were taken every 30seconds and recorded for both the adsorption and desorption steps.

During the adsorption step, the mass of methyl bromide loaded on thecarbon column was obtained by summing the flow rate provided by the massflow meters times the known feed concentration while the methyl bromidewas being fed to the column. During the desorption step, the mass ofmethyl bromide removed was obtained by summing the flow rate times theconcentration of methyl bromide provided by the IR analyzer, with thelatter being obtained every 30 seconds over 16 hours (ca. 1900 datapoints).

Temperature control of the column was not used during the adsorptionstep, such that the feed stream containing methyl bromide contacted theactivated carbon particles at ambient temperatures. During thedesorption step, the inlet and outlet temperatures were controlled towithin 98.5 to 101° C. by the recirculating pressurized hot watersystem. The desorption cycle time was approximately 4 times longer thanthe adsorption cycle time.

Table 1 shows the data measured for two corresponding adsorption anddesorption cycles.

TABLE 1 Run No. 1 Run No. 2 Adsorption Grams Methyl Bromide Loaded 7.787.73 Loading, % 2.50 2.48 Temperature, ° C. 16.2 17.8 Time, hours 4.04.0 Desorption Grams Methyl Bromide 7.50 7.49 Removed Temperature, ° C.98.5 101.0 Time, hours 16.2 15.1 Mass Balance (Out/In × 100) 96.4% 96.9%

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimswithout departing from the invention.

1. A method of removing an alkyl halide from a first gas volume, themethod comprising: contacting the first gas volume with an adsorbentcapable of adsorbing the alkyl halide from the first gas volume within afirst temperature range, thereby producing an alkyl halide-containingadsorbent; contacting a volume of air that is smaller than the first gasvolume with the alkyl halide-containing adsorbent within a secondtemperature range that is greater than the first temperature range andeffective to desorb at least a portion of the alkyl halide from theadsorbent, thereby producing a second gas volume containing the alkylhalide and having a volume less than the volume of the first gas volumeand an alkyl halide concentration greater than the alkyl halideconcentration of the first gas volume; and contacting the second gasvolume with a liquid phase comprising water and a nucleophile capable ofreacting with the alkyl halide to produce a purified gas stream.
 2. Themethod of claim 1, wherein the first temperature range is from −20 to35° C.
 3. The method of claim 1, wherein the second temperature range isfrom 80 to 120° C.
 4. The method of claim 1, wherein the secondtemperature range is not greater than 300° C.
 5. The method of claim 1,wherein the adsorbent comprises activated carbon particles.
 6. Themethod of claim 1, wherein the first gas volume is comprised of air andthe alkyl halide.
 7. The method of claim 1, wherein the volume of thesecond gas volume is less than 5% of the volume of the first gas volume.8. The method of claim 1, wherein at least 95% of the alkyl halidepresent in the first gas volume is adsorbed by the adsorbent.
 9. Themethod of claim 1, wherein the contacting of the first gas volume withthe adsorbent and the contacting of the volume of air with the alkylhalide-containing adsorbent are carried out within a vessel and thevessel is not transported between the contacting steps.
 10. The methodof claim 1, wherein the first gas volume is generated by providing anenclosure defining a confined space to be fumigated, placing a productwithin the enclosure, sealing the enclosure, introducing an amount ofthe alkyl halide effective to fumigate the product into the enclosure toproduce a gaseous atmosphere comprised of air and alkyl halide, allowingthe gaseous atmosphere to remain in contact with the product for alength of time effective to fumigate the product to a desired extent,and flushing the gaseous atmosphere from the enclosure using additionalair.
 11. The method of claim 1, wherein the nucleophile is selected fromthe group consisting of aliphatic thiols, aromatic thiols, salts ofaromatic thiols, salts of Is aliphatic thiols, aliphatic disulfides,aliphatic polysulfides, aromatic polysulfides, aliphatic polysulfides,sulfide anion, bisulfide anion, thiosulfate anion, sulfite anion,bisulfite anion, and thiocyanate anion.
 12. The method of claim 1,wherein the contacting of the second gas volume with the liquid phase iscarried out by bubbling the second gas volume into the liquid phase. 13.The method of claim 1, wherein the contacting of the second gas volumewith the liquid phase is carried out by passing the second gas volumethrough one or more gas dispersers having therein a plurality of holes,thereby producing gas bubbles, and passing the gas bubbles through theliquid phase.
 14. The method of claim 1, wherein the liquid phasefurther comprises between 1 and 25 wt % of an organic compound dissolvedtherein.
 15. The method of claim 14, wherein the organic compound is apolyethylene glycol.
 16. A method of removing an alkyl halide from afirst gas volume, the method comprising: contacting the first gas volumewith a first portion of an adsorbent capable of adsorbing the alkylhalide within a first temperature range, thereby producing a first alkylhalide-containing adsorbent; contacting a first volume of air that issmaller in volume than the first gas volume with the first alkylhalide-containing adsorbent within a second temperature range that isgreater than the first temperature range and effective to desorb atleast a portion of the alkyl halide from the first portion of adsorbent,thereby producing a second gas volume containing the alkyl halide andhaving a volume less than the volume of the first gas volume and analkyl halide concentration greater than the alkyl halide concentrationof the first gas volume; contacting the second gas volume with a secondportion of an adsorbent capable of adsorbing the alkyl halide within athird temperature range, thereby producing a second alkylhalide-containing adsorbent; contacting a second volume of air that issmaller in volume than the second gas volume with the second alkylhalide-containing adsorbent within a fourth temperature range that isgreater than the third temperature range and effective to desorb atleast a portion of the alkyl halide from the adsorbent, therebyproducing a third gas volume containing the alkyl halide and having avolume less than the volume of the second gas volume and an alkyl halideconcentration greater than the alkyl halide concentration of the secondgas volume; and contacting the third gas volume with a liquid phasecomprising water and a nucleophile capable of reacting with the alkylhalide to produce a purified gas stream.
 17. A system for removing analkyl halide from a gas volume, the system comprising: a) anadsorption/desorption unit comprising an adsorbent capable of adsorbingthe alkyl halide from the gas volume; and b) a reactor assemblycomprising a reaction vessel containing a liquid phase comprised ofwater and at least one nucleophile.
 18. The system of claim 17, whereinthe adsorption/desorption unit comprises a hollow column containing abed of the adsorbent in particulate form.
 19. The system of claim 17,wherein the adsorption/desorption unit and the reactor assembly areconnected by a line capable of transferring a gas stream from theadsorption/desorption unit to the reactor assembly.
 20. The system ofclaim 17, further comprising an enclosure defining a confined space andcapable of being sealed, the enclosure being connected to theadsorption/desorption unit by a line capable of transferring a gasstream from the enclosure to the adsorption/desorption unit.
 21. Thesystem of claim 17, wherein the reactor assembly further comprises oneor more gas dispersers having therein a plurality of holes, the one ormore dispersers being immersed in the liquid phase.